Transmission Modes

Different ways of characterizing the transmission

Timing of the transmission of the data bits

• Serial– Data bits transmitted at

different times– One bit after the other

• Parallel– Multiple bits transmitted

simultaneously (same time)– Typically with different data

“lines” for each bit

01011

1 1 0 1 0

01011

01011

Timing between transmitter and receiver

• All transmissions are synchronized somehow– once per bit (Manchester)– once per byte – once per frame …..

• Asynchronous (means without synchronization) but DOES synchronize once per BYTE.

• Awful name

ASYNCHRONOUS

Serial (asynchronous) Encoding

0

StartBit IdleIdle

1

Stop Bit

FIRSTLAST

0 0001 1 1 1

DataParity

0

Idle -> No information on the lineStart Bit -> Defines the beginning of the byteData -> Information (number of bits varies)Parity -> A check digit for correct reception (more later) Even/Odd/NoneStop Bit-> A check for correct detection of start bit 1/1.5/2 bits long

0

StartBit IdleIdle

1

Stop Bit

0 0001 1 1 1

DataParity

0

Start Bit Timing

Bit Centers

Clock -> 4 times faster that bit rate 2 ticks from beginning is bit center 4 ticks from there is next bit center

Parity

• Counts number of ones in DATA

• Sets the parity bit to 1/0 – Even or – Odd

• May not choose to use at all (None)

• Not a good means of error detection

• Error in one bit 10-6 … Error in 2 bits 10-12

• Assumes independence of bit errors … not always true

Parity examples

DATA

0 0 1 0 0 1 0 0 1 1 1 0 0 0 1 1

PARITY (even)

0

1

0 + 2 = 2

1 + 5 = 6

Use Second example and assume errors1 0 1 0 0 0 1 1

1 0 0 0 0 0 1 1

1

1

1 + 4 = 5 ERROR

1 + 3 = 4 ???????

One can’t detect multiple bit errors properly!

Serial TransmissionMany concepts in one byte

• Synchronization on a byte level

• Framing with start and stop bit

• Error detection with parity

• What does this cost us?

0

StartBit IdleIdle

0 0001 1 1 1

DataParity

01

Stop Bit

Efficiency

Data

Data + OverheadEfficiency = =

8

1 + 1 + 8 + 1 =

8

11 = .7272

1200 bps line modem = 1200 * .7272 = 872 bps ignoring idle!

Where would you see it?

• On a PC it is the COM1, COM2 .. Port• Typically RS232 interface

– 9 pin– 25 pin – or others

• Modem, mouse, keyboard• ASYNCHRONOUS because one can’t tell when

the data will be transmitted from one byte to the next

Serial Summary

• Same name (asynchronous) used for two concepts– lack of timing– Serial (byte transmission)

• NOTHING in the name imples BYTE transmission but that is how it is used

• Synchronizes once per byte– assumes clocks will remain synchronized until the end

of the byte

• Illustrates OVERHEAD

So what is Synchronous?

• Synchronizes – once per block of data not per byte

• Typically faster rates• USB ports on a PC (find rates on www)

– see www.pcs.cnu.edu/~dgame/cs335/topics/usb.ppt

– easier to understand after protocols

• More complex framing (each of these are bytes typically)

(end) errordetect DATA control sync sync

Sync byte/string

• A pattern with which receiver can established synchronization

• The longer it is (to a point) the greater the reliability of the synchronization

• Like a start bit• 010101010101• No idle times between bytes(bits) in the

frame.

Isochronous• Asynchronous

– irregular gaps between bytes

• Synchronous – no gaps between bytes

– gaps between blocks

• Isochronous– REGULAR gaps between blocks

– telephone PCM

– 4000Hz -> 8000 samples/sec -> 8 bits/sample-> 64000 bps

– What if on 1.5 Mpbs line?

AsynchronousSynchronousIsochronous

Different arrival rates of bytes

Alternating Interactions

Device 1 Device 2

time

data

Simplex - one way(tv,radio, weather satellite)

Device 1 Device 2

time

data

Half Duplex - alternate each way(telephone, cb, ham radio)

Device 1 Device 2

time

data

Full Duplex - both ways same time(computer serial)

Sharing the medium

Many users

One channel

Multiplexing

• Space - division– physically separate channels (wires)

• Time - division– sharing a CPU in multiprogramming OSs– telephone connections to a switching station

• Frequency - division– tv channels on a cable line– telephone conversations on a TRUNK line– radio stations sharing the airwave

Space division

User 5

User 6

User 7

User 8

User 1

User 2

User 3

User 4

Time and Frequency division

User 5

User 6

User 7

User 8

User 1

User 2

User 3

User 4

Medium

Time Division

time

frequencyUser 1 and User 5User 2 and User 6User 3 and User 7

Frequency Division

time

frequencyUser 1 and User 5User 2 and User 6User 3 and User 7

Statistical Multiplexing

• Making the use of the medium more efficient• Examples

– cars on the highway– seats reserved on an airline flight– lines for making phone calls

• All overbook. Do not provide sufficient capacity to meet maximum demand.

• Provide less capacity. Save money. Usually good enough!

Data Transmission ExampleTDM

SITE

1

SITE

2

A4A3A2A1

B4B3B2B1

C4C3C2C1

D4D3D2D1

..A2A1

..B2B1

..C2C1

..D2D1

D3C3B3A3D4C4B4A4...

Fully Utilized!

Data Transmission ExampleNOT Fully Utilized (9/16)

SITE

1

SITE

2

A4A3…A1

B4….B2B1

C4…….C1

……….D1

…..A1

..B2B1

…..C1

…..D1

……….A3...C4B4A4...

How Do We Make This More Efficient?

Under-Allocate

SITE

1

SITE

2

A4A3…A1

B4….B2B1

C4…….C1

……….D1

…..A1

..B2B1

…..C1

…..D1

A30001C4B4A40111...

4 bits overhead per frame saves wasted slots.Less capacity required. Unable to meet Maximum Demand.

Overhead

Queueing

• Statistical multiplexing generates a whole new science

• Underallocating generates potential waiting lines– gas station– bank tellers– on-ramps at interstate– your personal “to-do” list …………….

• Computer simulation– when to change resource amount (more tellers)

Multiplexing a subtle distinction

• Users trying to make calls– Statistical– Some users have to wait to gain access

• Calls actually on the line– Not Statistical– Once on, you consume the line as long as you

are connected